Conventionally, a PS plate comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer has been widely used as a lithographic printing plate. According to the usual production method therefor, a plate is subjected to mask exposure (plane exposure) through a lith film and then the non-image area is dissolved and removed to obtain a desired printing plate.
In recent years, digitization technology of electronically processing, storing and outputting image information using a computer has been widespread and various new methods for outputting an image, which can cope with this technology, have been used in practice. This tendency has brought about the demand for a computer-to-plate (CTP) technique of requiring no lith film but directly producing a printing plate by scanning highly directive light such as laser light according to digitized image information. As a result, it has become an important technical concern to obtain a printing plate precursor suitable for the plate-making by CTP.
As one of the systems for obtaining a lithographic printing plate capable of scan exposure, a constitution such that a photopolymerizable composition having an excellent photosensitive speed is used for the ink-receptive photosensitive resin layer (hereinafter referred to as a “photosensitive layer”) formed on a hydrophilic support has been heretofore proposed and already put into market. The printing plate precursor having such a constitution enables simple and easy development and further assures preferred plate and printing performances such as excellent resolution, inking property, press life and scumming resistance.
The above-described photopolymerizable composition fundamentally comprises an ethylenically unsaturated compound, a photopolymerization initiation system and a binder resin, and the image formation proceeds as follows. The photopolymerization initiation system absorbs light to produce an active radical and this induces addition polymerization of the ethylenically unsaturated compound, as a result, the photosensitive layer is insolubilized.
In most of conventional proposals on the photopolymerizable composition capable of scan exposure, use of an initiation system having excellent photosensitivity is disclosed. A large number of such systems are described, for example, in “Bruce M. Monroe et al., Chemical Revue, Vol. 93, pp. 435-448 (1993)” and “R. S. Davidson, Journal of Photochemistry and Biology A: Chemistry, Vol. 73, pp. 81-96 (1993)”.
As for conventional CTP systems using a photopolymerization composition comprising such an initiation system and employing a long-wavelength visible light source such as Ar laser (488 nm) and FD-YAG laser (532 nm), writing at a higher speed is demanded so as to elevate the productivity in the plate-making process. However, this requirement is not yet satisfied, because the output of light source or the sensitivity of photosensitive material is not sufficiently high.
On the other hand, for example, a semiconductor laser using an InGaN-type material and being capable of continuous oscillation in the region from 350 to 450 nm has recently come into practical use. The scan exposure system using such a short-wave light source is advantageous in that the semiconductor laser can be produced at a low cost in view of its structure and an economical system can be established while ensuring a sufficiently high output. Furthermore, as compared with systems using an FD-YAG or Ar laser, a photosensitive material having sensitivity in a short-wave region and in turn enabling operation under brighter safelight can be used.
However, a photoinitiation system having satisfactory sensitivity for scan exposure in a short-wavelength region of 350 to 450 nm is not yet known at present.
The technique for obtaining a high-sensitivity photoinitiation system is still widely demanded in the imaging field (see, for example, JP-A-2000-258910 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), “J. P. Faussier, “Photoinitiated Polymerization—Theory and Applications”: Rapra Review, Vol. 9, Report, Rapra Technology (1998)” and “M. Tsunooka et al., Prog. Polym. Sci., 21, 1 (1996)”). A photoinitiation system comprising a sensitizing dye and an activator can generate an acid or a base other than the above-described active radical by selecting the activator and this system is used, for example, in the image formation such as stereolitlography, holography and color hard copy, in the electronic material production field such as photoresist, and in the usage for photocurable resin materials such as ink, paint and adhesive. In these industrial fields, for inducing decomposition of the activator with good efficiency, it is demanded to find out a sensitizing dye excellent in light absorptivity and sensitization ability.
With respect to the technique for prolonging the press life of the printing plate, attempts have been made to use a high-strength urethane polymer as the binder (see, for example, JP-A-3-287165), but satisfactory press life is not obtained. A lithographic printing plate satisfied in all of image formability, scumming on printing and press life is demanded.